Re: [xfsprogs PATCH 2/2] xfs_io: add a new 'log_writes' command

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Fri, Nov 17, 2017 at 03:03:39PM -0600, Eric Sandeen wrote:
> On 11/17/17 2:48 PM, Ross Zwisler wrote:
> > On Fri, Nov 17, 2017 at 02:39:07PM -0600, Eric Sandeen wrote:
> >> On 11/17/17 2:25 PM, Ross Zwisler wrote:
> >>> Add a new 'log_writes' command to xfs_io so that we can add dm-log-writes
> >>> log marks via the external 'dmsetup' executable.  It's helpful to allow
> >>> users of xfs_io to adds these marks from within xfs_io instead of waiting
> >>> until after xfs_io exits because then they are able to replay the
> >>> dm-log-writes log up to immediately after another xfs_io operation such as
> >>> mwrite.  This isolates the log replay from other operations that happen as
> >>> part of xfs_io exiting (file handles being closed, mmaps being torn down,
> >>> etc.).  This also allows users to insert multiple marks between different
> >>> xfs_io commands.
> >>>
> >>> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> >>> Suggested-by: Dave Chinner <david@fromorbit.com>
> >>
> >> Without reviewing in detail, what is the advantage of wrapping dmsetup
> >> into xfs_io?  My first inclination is that there is none at all, and
> >> xfstests can call dmsetup as easily as they can call xfs_io.  No?
> >>
> >> -Eric
> > 
> > I commented on this a bit in the changelog for the 2nd patch:
> > 
> > It's helpful to allow users of xfs_io to adds these marks from within xfs_io
> > instead of waiting until after xfs_io exits because then they are able to
> > replay the dm-log-writes log up to immediately after another xfs_io operation
> > such as mwrite.  This isolates the log replay from other operations that
> > happen as part of xfs_io exiting (file handles being closed, mmaps being torn
> > down, etc.).  This also allows users to insert multiple marks between
> > different xfs_io commands.
> > 
> > I agree that the shell-out to dmsetup isn't awesome...  For the current test I
> > have written I think we can get away with just assuming that the xfs_io exit
> > stuff won't interact too heavily with the dm-log-writes log, and we could
> > potentially move the dmsetup call back into the fstest.  This is how I
> > initially had it, and moved it into the C program via shell-out in response to
> > Amir's feedback:
> 
> Sorry, terrible of me to not have read that.  :(  Ok, so next question - 
> DM_TARGET_MSG seems to be public, can we just invoke the ioctl directly
> instead of shelling out to dmsetup?
> 
> I'm checking w/ the dm folks too, to make sure that's expected to work.  As
> long as the use isn't too tricky it seems like that might be better.

Yea, that seems like a better option - I'll take a look.  Thanks for the
suggestion.